We know that,

Gravitational force F = G (m₁m₂/r²)       …(i)

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According to question,

if the distance between them is reduced to half, then,

r¹ = r/2

∴ F¹ = G (m₁m₂/(r/2)²) 

F¹ = (Gm₁m₂/r²).4         …(ii)

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From equation (i) and (ii)

F/F¹ = 1/4

⇒ F¹ = 4F

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Thus the force of attraction between them increases to four times.